Wireless water heater monitor uses whatever was lying around

[Chris] set out to build a monitoring system for his water heater. It doesn’t Tweet or send SMS messages. It simply lights up an LED when the water heater is active. The one thing that complicates the setup is that he didn’t want to pull any wire from the garage into the house. What you see above is the wireless setup he used to accomplish this goal.

This is an electric water heater, so [Chris] patched into the 230V heating element feed. When the water heater is idle this connection is cut off. He used a transformer to step the voltage down to 17V and rectified it before feeding a 7805 power regulator. The rest of the transmitter circuit consists of a 555 timer driving the coil seen on the left. It is made out of telephone wire, with each of the four conductors inside connected together to multiply the number of windings. The box of breakfast sausages hosts the receiver coil. His hardware takes the induced current from that coil and amplifies it, feeding the signal to the base of a transistor responsible for switching the status LED. This works through the 6″ thick garage wall, although he did have to use a battery on the receiving end as his wall wart was injecting way too much noise into the system to work.

So in the transmitter circuit, the {L1, D2, D1} convert the 5V DC to AC from the 10KHz 555 signal? I have always had the notion that oscillations required an inductor / capacitor combination. Is the zener assisting this transformation?

Also, I am guessing that the only reason he had to use a 555 is to get a higher frequency that would pass through a wall as opposed to the 50hz?

a simpler way maybe to use 2 tv coils (if you have taken apart 2 color tvs) that has a neck coil (sony winds it around a plastic ring attached to yoke others it is a 5 or 6 inch diameter ring of very thin wire)

and wire a resistor to limit the current that is the transmitter.

the receiver just a resistor to limit the the current and a diode to make dc and an led.

you may need a smaller coil for the receiver since you may get 110 to 220 out of the receiver.

you are right about not wanting to drill holes since if the house is a rental unit the land lord may not like holes or may take your security deposit to repair holes.